1. Field of the Invention
The present invention relates to a novel placido pattern. More specifically, the novel placido pattern of the present invention allows for easier, more accurate, and more economical detection of concentric anomalies that may occur on the eye being examined.
2. Description of Related Art
Placido imaging and placido examinations commonly referred to as keratometry date back over 150 years. Traditionally keratometry enables measurements of eye curvature that depend on the fixturings used, as well as set-up geometry. Using keratometry, the geometry of an eye can be derived manually by examination of a photograph. A reference placido pattern is first captured photographically after being reflected off of a sphere of known radius. A fundamental theorem of placido geometry states that the magnification of the reflected placido image is directly proportional to the radius of curvature of the examined object. Traditional patterns typically employed a series of concentric light and dark circles, such as shown in FIG. 1.
The concentric ring pattern of FIG. 1 developed by Placido was developed on the premise that the concentric ring patterns reflected from the human cornea would distort based on the anterior shape of the examined cornea. In the case of radical change of curvature of the cornea, such as a smaller radius or more curvature, the rings would appear to be further apart. For areas that are larger in radius, the rings would appear to be closer together. For a perfectly spherical cornea, the rings remain concentric and evenly spaced.
Within the last twenty years or so, it has been found that if a recording device, such as a camera, captures these images, the captured images can be compared to an image from the reflection of a close-to-perfect reference sphere. The differences between the two images then indicate how much the curvature of the anterior surface of the examined eye has changed from a perfect sphere. Other prior art placido patterns include a spider-web pattern, which is fully described in U.S. Publication 2004-0061833 and is commonly assigned with the present invention to Bausch & Lomb Incorporated. An example of such spider-web pattern is shown in FIG. 2. FIG. 3 shows yet another prior art placido patter which may be referred to as a dartboard pattern. The dartboard pattern can be said to be a combination of concentric and radial edges.
A camera typically is placed behind a hole in the center of the placido pattern and is aimed toward a patient's eye. During an acquisition of an image of the eye with a reflected placido pattern, the device is placed in an appropriate location in front of the eye. The surface of the cornea of the eye being examined then reflects the pattern and the images captured by the camera placed behind the placido pattern. The reflection of the pattern will change or distort depending on the changes in curvature on the corneal surface from that of a perfect sphere. With certain image processes and analysis software in known instruments, such as the Bausch & Lomb Incorporated Orbscan II™ Systems or other known keratometors or topography systems, it is possible to build a curvature map of the anterior surface of the cornea being examined.
A major problem with the ringed placido of FIG. 1, is the possibility of acquiring ambiguous data. If a curvature anomaly occurs, tangentially to a ringed edge from the X-Y perspective of the eye image, an ambiguity may occur. This is referred to as a concentric anomaly. Another type of anomaly that may be found is a radial anomaly. A radial anomaly is detected from the reflection of the placido rings.
However, if specific points are added to the ring pattern in the middle of the rings, it is much easier to detect a concentric anomaly. The spider-web placido pattern of FIG. 2, does not share the limitation on detecting concentric anomalies with the pattern of FIG. 1. However, the algorithm required to find the edges in a spider-web placido pattern is very complex relative to the algorithms used for a simple placido pattern of FIG. 1. This is especially true since the concentric edges in the spider-web pattern are interrupted by radial lines or edges, which corrupt some of the relevant data with regard to the concentric edges.
The dartboard placido of FIG. 3 also does not have the concentric anomaly limitation of the simple ring placido. However, like the spider-web placido pattern, a more complex edge detection algorithm is required. In addition, the alternating black and white blocks do not produce smooth single edges, especially if the placido is slightly out of focus.
Therefore, it would be advantageous to have a placido pattern which can detect concentric anomalies relatively easily with the use of less complex edge detection algorithms required of some prior art patterns.